The enhanced catalytic activity and stability of oxodiperoxomolybdenum- modified mesoporous organosilicas in selective epoxidation reactions

Article abstract of DOI:10.1039/b901600g

Complexes of the type (L-L)MoO(O₂)₂ heterogenized on phenylene-bridged mesoporous organosilicas show a 10-fold increase in catalytic activity and a high stability in liquid phase epoxidation reactions, with H ₂O₂ as the oxidant, compared to the corresponding MCM-41-derived systems.

Full text of DOI:10.1039/b901600g

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LETTER
www.rsc.org/njc | New Journal of Chemistry
The enhanced catalytic activity and stability of oxodiperoxomolybdenum-
modified mesoporous organosilicas in selective epoxidation reactionsw
a
b
c
a
Sankaranarayana pillai Shylesh, Mingjun Jia, Andreas Seifert, Sridhar Adappa,
a
a
S. Ernst and Werner R. Thiel*
Received (in Montpellier, France) 26th January 2009, Accepted 9th February 2009
First published as an Advance Article on the web 17th February 2009
DOI: 10.1039/b901600g
Complexes of the type (L–L)MoO(O2)2 heterogenized on
phenylene-bridged mesoporous organosilicas show a 10-fold
increase in catalytic activity and a high stability in liquid phase
For this purpose, catalysts of the type (L–L)MoO(O ) were
2 2
tethered onto hydrophobic PMOs derived from phenylene
(–C H –, B)- and ethylene (–CH CH –, E)-bridged organosilicas.
6
4
2
2
epoxidation reactions, with H
2
O
2
as the oxidant, compared to
PMOs B and E were prepared from their corresponding
bifunctional silsesquioxane precursors. Molecular organization
in the walls is an outstanding feature of B since, in this
case, hydrophobic arene and hydrophilic silica layers are
˚
alternatively arranged with a periodicity of 7.6 A. The
molybdenum-containing precursors were synthesized as
4
outlined in Scheme 1. Mesoporous catalysts BMo, EMo
the corresponding MCM-41-derived systems.
Selective olefin epoxidation generates valuable starting
materials for further functionalization, and opens up access to
industrially important products such as polyethers, epoxy resins
6
1
and surfactants. A whole series of homogeneous catalysts have
been identified as being active in this reaction. Among them,
molybdenum- and tungsten-based catalytic systems play an
and MMo (catalyst on conventional MCM-41) were prepared
by following a post-synthetic grafting method in dry toluene
using supports B, E and M.w
2
important role. However, the regeneration of the homo-
geneous catalyst from reaction mixtures causes cost and waste
problems. It is therefore worthwhile developing innovative
approaches for the heterogenization of the homogeneous cata-
lysts for this reaction using environmentally benign oxidants.
Catalyst heterogenization on solid supports with high
surface areas is a convenient method, since the unique textural
features of porous materials allow high catalyst loadings and
The success of the grafting procedure could be demonstrated
1
3
spectroscopically.w All the heterogenized catalysts showed
C
CP-MAS NMR spectra similar to those of their free congener.
Typically, the resonances of the two carbon atoms of the
ethoxide groups (57.7 and 14.9 ppm) decreased in intensity due
to condensation with the surface Si–OH groups and liberation
4
13
of EtOH. Besides, the C CP-MAS NMR spectra of BMo
(EMo) showed sharp peaks at 133.7 (5.3) ppm, corresponding
3
the uniform dispersion of active sites. Recently, we verified
the application of molybdenum complexes of the type
6 4 2 2
to the –Si–C H –Si– (–Si–CH –CH –Si–) groups in the
2
framework walls. The Si MAS NMR spectrum of BMo
9
(
L–L)MoO(O
onto mesoporous MCM-41 for the epoxidation of cyclooctene,
2
)
2
(L–L = pyrazolylpyridine ligand) grafted
2
displayed three signals, which could be assigned to the T
3
t
4
with BuOOH as the oxidizing agent. We proved that
removing the residual Si–OH groups on the support by
silylation enhanced the stability of the material and facilitated
the regeneration of the catalyst due to the increased surface
and T silicon centres bound to phenylene (À81.4 and
À71.6 ppm), and to the linker unit of the molybdenum
7
complex (À71.6 and À63.5 ppm). EMo showed two peaks:
2
one at À58 ppm (T : C(OH)Si(OSi)
2
) and the other at
4b
3
hydrophobicity. However, such systems still show only poor
catalytic activity with H O as the oxidant. It therefore seemed
À67 ppm (T : CSi(OSi)
3
). There were no resonances typical
n
of Q species (Q = Si(OSi)
n
8
n
(OH)4Àn, n = 2–4), which means
that no silicon–carbon bond cleavage occurred during the
2
2
to be beneficial to investigate the heterogenization of such
complexes on so-called ‘periodic mesoporous organosilicas’
synthesis of the supports and the subsequent high temperature
2
9
(
PMOs). PMOs are available via
condensation of bifunctional precursors of the type
RO) Si–X–Si(OR) . They have a homogeneous distribution
a
template-directed
grafting process. The Si MAS NMR spectrum of MMo, as
expected, showed signals at À110 to À90 ppm (Q sites) and
(
3
3
of organic fragments and silica moieties in the framework. It is
known that hydrophobic surfaces can improve the catalytic
5
activity and stability of active sites in inclusion chemistry.
a
Fachbereich Chemie, TU Kaiserslautern, Erwin-Schro¨dinger-Strabe
Geb. 54, 67663 Kaiserslautern, Germany.
E-mail: thiel@chemie.uni-kl.de; Fax: +49 631 2054647;
Tel: +49 631 2052752
College of Chemistry, Jilin Uinversity, 130023 Changchun, PR China
Institut fu¨r Chemie, TU Chemnitz, Straße der Nationen 62,
b
c
Scheme
1
Synthesis of oxodiperoxo{(3-triethoxysilylpropyl)-
09111 Chemnitz, Germany
[
3-(2-pyridyl)pyrazol-1-yl]acetamide}molybdenum; i: THF, NaH,
w Electronic supplementary information (ESI) available: Experimental
details, C CP-MAS NMR, etc. See DOI: 10.1039/b901600g
13
2 2 2 3 3 2 2 2 2 2
BrCH COOEt, ii: NH (CH ) Si(OEt) , iii: CH Cl , (dmf) MoO(O ) .
This journal is ꢀc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
New J. Chem., 2009, 33, 717–719 | 717

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Fig. 3 &: Kinetic profiles for the epoxidation of cyclooctene with
2
9
Fig. 1 (A) Si CP-MAS NMR spectra and (B) N
2
adsorption/
t
7
0% BuOOH in H
ing the reaction after removing the catalyst after 1 h. J: Kinetic
profile with H as the oxidizing agent.
2
O. n: Heterogeneous reaction check by continu-
desorption isotherms at 77 K of (a) BMo, (b) EMo and (c) MMo
samples.
2 2
O
weak resonances at À70 to À60 ppm (T sites), which originated
or MMo, although the catalyst loading was similar for all
samples (Fig. 3).
from the low loading of the silylating agents (Fig. 1A). N
2
adsorption/desorption experiments gave type IV isotherms,
with a sharp capillary condensation step at P/P = 0.3–0.4 for
Furthermore, we recently reported that homogenous
o
(
L–L)MoO(O
2
)
2
catalysts, as well as MCM-41-grafted
all samples, typical of mesoporous materials having a uniform
mesopore structure (Fig. 1B). The measured BET surface
areas and BJH pore sizes of the BMo, EMo and MMo samples
samples, show only poor cyclooctene conversion with 30%
4
H
2
O
2
as the oxidizing agent. In contrast, BMo/H
cyclooctene in about 60% yield after 6 h, while EMo gives only
0% and MMo shows an almost negligible conversion of 6%.
Therefore, the nature of the support is responsible for a
0-fold enhancement of the catalytic activity. This effect,
2 2
O converts
2
À1
were 613, 595 and 634 m
g
, and 2.5, 2.4 and 2.4 nm,
respectively. Furthermore, the catalyst loading of all the
3
mesoporous materials, determined by their nitrogen content,
À1
1
was found to be nearly 0.4 mmol g
.
t
which is also observed for 70% BuOOH in H O, can, in
2
Powder XRD patterns of BMo, EMo and MMo showed
typical peaks at low angles related to the hexagonal mesoporous
9
structure. Additional peaks appeared for BMo in the range
our opinion, be assigned to the increased hydrophobicity of
the framework walls due to the presence of phenylene bridges
in defined positions. The unique hydrophobic pores facilitate
the adsorption of olefins close to the active sites, and/or reduce
1
01 o 2y o 501 that could be assigned to a periodicity with a
spacing of 7.6 A in the framework walls (Fig. 2).
6
˚
the adsorption of the more polar epoxide and by-products
t
(
2
BuOH or H O). Encouraged by the results with cyclooctene,
the epoxidation of a series of other olefins was investigated
under optimized reaction conditions (Table 1). Similar to
cyclooctene, BMo converts these olefins to their corresponding
epoxides in good to excellent yields, showing again the novelty
of the present catalytic system.
Additionally, the catalytic properties of (L–L)MoO(O2)2
grafted onto hydrophobic PMOs remained unchanged upon
reuse. The materials turned out to be recyclable up to four
times, while samples with conventional MCM-41 show a
continuous decrease in catalytic activity after each run.
Experiments where the solid catalysts were removed at the
Fig. 2 Powder XRD patterns of (a) MMo, (b) EMo and (c) BMo.
Table 1 The epoxidation reaction of different olefins catalysed by
BMo
a
The molybdenum-containing samples were tested in the
epoxidation reaction of cyclooctene at 60 1C, in the presence
b
Yield of epoxide (%)
t
of 1 mol% of catalyst, with different solvents (CHCl
3
, toluene,
O, H ).
As expected, the yield of the reaction was both solvent- and
Entry
Substrate
BuOOH
H O
2
2
t
CH
3
CN) and oxidizing agents (70% BuOOH in H
2
2 2
O
1
2
3
4
5
Cyclooctene
Cycloheptene
Cyclohexene
Styrene
100
96
85
83
73
62
55
49
48
34
t
3
oxidant-dependent; a combination of CHCl and BuOOH
gave the best conversions, with nearly 100% selectivity for
epoxycyclooctane,w corroborating with our observations of
homogeneous epoxidation catalysis using complexes of the
1-Octene
a
Reaction conditions: 1 mmol alkene, 1.2 mmol oxidant, 1 mol%
b
1
0
BMo, 5 mL CHCl
3
, 6 h, reflux. Yields were determined by GC-MS
type (L–L)MoO(O
2
)
2
.
However, under identical reaction
with respect to an internal standard (n-decane).
conditions, BMo showed a much higher activity than EMo
7
18 | New J. Chem., 2009, 33, 717–719
This journal is ꢀc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009

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reaction temperature after 1 h support this conclusion (Fig. 3).
After this time, there was no catalytic activity in the solutions
atmosphere of nitrogen for 12 h at 90 1C. The solid was
filtered off, washed with CHCl and dried in vacuo to obtain
the mesoporous supported hybrid materials.
3
t
for BMo and EMo (with 70% BuOOH in H O), while for
2
MMo, leaching of the catalyst had taken place. We assign this
behavior to the coordinatively-inert framework walls, which
make the ligand–molybdenum interaction become dominant.
In summary, the present studies show the first example of
the catalytic activity and stability tuning of a heterogenized
epoxidation catalyst by the nature of its support. Compared
to conventional MCM-41 or its surface silylated analogs,
hydrophobic mesoporous organosilica materials give an
outstanding activity, along with high stability and selectivity,
even in the presence of cheap and environmentally benign
aqueous oxidants. The results further confirm that the use
of structurally well-defined PMOs as supports allows the reuse
of the catalysts without any loss of activity, which is of
special importance for expensive noble metals and/or chiral
ligands. Further investigations of the surface properties of
these materials is currently under way to understand their
relationship with the catalytic properties.
Powder X-ray diffraction patterns were obtained on a
Siemens D5005 diffractometer with Cu-Ka radiation
(30 kV, 30 mA). Nitrogen adsorption/desorption isotherms
were measured at 77 K on a Quantachrome Autosorb
1 sorption analyzer after evacuation of the samples at
1
3
29
150 1C overnight. Solid-state C and Si CP-MAS NMR
spectra were recorded at 100.6 and 79.49 MHz, respectively,
using a Bruker AVANCE 400 spectrometer.
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1
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1
A portion of 0.03 g of oxodiperoxo{(3-triethoxysilylpropyl)-
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1
This journal is ꢀc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
New J. Chem., 2009, 33, 717–719 | 719